PSI - Issue 23

M. Rogante et al. / Procedia Structural Integrity 23 (2019) 95–100

96

M. Rogante et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

Nitinol, a thermoelastic NiTi shape memory alloy with approximately 50 at. % Ti, is adopted in a wide range of medical equipment and devices used in interventional radiology, orthopedics, neurology and cardiology, in particular as a smart material for stents. Fracture occurrences of up to 50%, probably due to in vivo cyclic displacements, have been reported in a number of Nitinol stents after one year. Knowledge of nanostructure of this material, thus, is fundamental to understand the damage micro-mechanisms, especially with reference to the consequences of the geometrically reversible stress-induced martensitic phase transformation on crack-growth resistance; Rogante et al. (2011a); Rogante et al. (2011b). A novel surface engineering technique called Powder Immersion Reaction Assisted Coating (PIRAC) can produce well-adherent, uniform TiN coatings on NiTi products. It is a simple diffusional process which is based on the immersion of samples into powders of an unstable nitride (such as Cr 2 N), and their subsequent annealing at high temperatures in chromium-rich stainless steel foil bags; Zorn et al. (2008) and Starosvetsky & Gotman (2001). No work has been made to characterize PIRAC using Neutron characterization techniques. In the present work, Small Angle Neutron Scattering (SANS), Ultra-Small Angle Neutron Scattering (USANS) and High-Resolution Neutron Diffraction (HRND) have been adopted.

Nomenclature A

parameter related to the scattered intensity

D I 0

slope of scattering function

zero angle scattering intensity, arb. un.

P(R)

distributions of the distances momentum transfer, Å -1

Q R R c R g r g

pores size, nm

correlation radius, nm gyration radius, nm

gyration radius of the system of defects, nm

Δω

rocking angle

2. Materials and samples

NiTi coupons (Brindley Metals Ltd, UK) of 25.4 mm in diameter were polished to a mirror finish and ultrasonically cleaned in isopropyl alcohol. The samples were subjected to a treatment called Powder Immersion Reaction Assisted Coating (PIRAC) at 900ºC for 1.5 hours and 3 hours followed by a water quench. Treatment details can be found in the paper by Starosvetsky & Gotman (2001). However, a number of modifications were introduced in the treatment presented in this work. The prepared PIRAC bags were each inserted into similar bags of a larger size for additional protection from atmospheric oxygen. Additionally, Zr pellets (getter) were also added in both the inner and outer bags to act as an oxygen scavenger. The getter had a surface area of 45 cm 2 and of 60 cm 2 in the inner and outer bags respectively. The analyzed NiTi samples of Fig. 1 were: A) massive, untreated; B) massive, submitted to PIRAC at 900°C for 1.5h and water quenched - coating thickness: TiN (0.1 μ m) + Ti2Ni (0.6 μ m); C) massive, submitted to PIRAC at 900°C for 3h and water quenched - coating thickness: TiN (0.5 μ m) + Ti2Ni (0.6 μ m); D, E) commercial stents. Samples A, B and C have been produced by the Department of Metallurgy & Materials Engineering of the University of Malta. Samples D and E have been supplied by the Neuroradiology Dept., Azienda USL, Emilia Romagna Region Health Service, Italy.